The present invention relates to a device for Electron Paramagnetic Resonance Imaging. The device of the present invention has potential use in one-, two- or three-dimensional Electron Paramagnetic Resonance (EPR) Imaging. It has potential application in leather industry for detecting the presence and distribution of chromium in leather. The device may also be used in the drugs, pharmaceuticals and cosmetics industry. It is also envisaged to have use for detecting the presence and distribution of radicals including the nitroxide radical in metabolites for medical purpose. Further, it has potential application in mineralogical applications and in foodstuffs industry.
Electron Paramagnetic Resonance (EPR), which is also known as Electron Spin Resonance (ESR), is a technique of radiofrequency (rf)/microwave spectroscopy that detects free radicals, certain transition metal complexes, certain rare earth metal complexes, triplet state molecules and the like, by virtue of the presence of unpaired electrons in such species (xe2x80x98spinsxe2x80x99). The measurement is performed by locating the sample of interest (usually liquid or solid) in a suitable resonator (e.g. cavity, slow wave structure, etc.) placed in a spatially homogeneous magnetic field and irradiating it with electromagnetic (em) radiation whose frequency matches the characteristic precession frequency of the electron spins in the external field, the resonance frequency being given by the equation       ω    0    =                    g        ⁢                  xe2x80x83                ⁢        β            ℏ        ⁢          B      0      
where g is the spectroscopic splitting factor (Lande factor, or simply the g-factor), xcex2 is the Bohr magneton, B0 is the intensity of the magnetic field and  is Planck""s constant divided by 2xcfx80. This xe2x80x98first orderxe2x80x99 equation for the resonance frequency is usually modified by the addition of other contributory terms involving internal (or local) fields from other unpaired electrons and/or nuclear spins. The resonant absorption (or emission) of em radiation under these conditions is customarily recorded by varying the intensity of the external magnetic field across the resonance condition, holding the frequency of the em radiation constant in continuous wave (cw) mode. In order to minimize dc (direct current) drifts during the course of the field scan and to improve signal-to-noise ratio, it is customary to employ field modulation, typically at 100 Hz to 100 kHz, employing a set of modulation coils mounted in the resonator/cavity and perform phase sensitive detection (PSD) of the output of the em detector (e.g. Diode detector). Normally, EPR measurements are employed to access detailed information about the electronic structure, shape and dynamics of molecular species, the sample and magnetic field being made as homogeneous as possible. The magnetic field is usually generated by a permanent magnet, superconducting electromagnet, or, more frequently, by a resistive electromagnet. The intensity of the field generated by the magnet is usually not higher than 1.5 Tesla (T), although very low fields, of the order of 0.01 T, or very high fields, of the order of 3.4 T may also be employed. Correspondingly, the sample placed in the resonator is irradiated typically with microwaves from a microwave bridge, or with radiofrequency radiation.
In the pulsed mode of operation, the continuous wave (cw) microwave bridge is replaced with a pulsed microwave bridge, the microwaves are amplified with a Traveling Wave Tube (TWT) Amplifier to produce typically upto 1 kWatt of power in pulsed output mode, such microwave pulses then being fed to the resonator. Often a resonator with lower quality factor, Q, is used for pulsed applications, in the interest of reducing the ring-down time (xe2x80x98dead timexe2x80x99) of the system, to enable near xe2x80x98zero timexe2x80x99 detection of the resulting ESR signals.
The same basic experiment may be used in quite a different mode, however, to detect macroscopic molecular distributions in an inhomogeneous object. Such a distribution function provides an xe2x80x98imagexe2x80x99 of the object in question. The primary emphasis of such a measurement is not molecular structure, but macroscopic molecular distribution. Locating the object of interest in a suitable resonator in a magnetic field that has spatial variation, i.e., gradients, accesses this type of information. Under these conditions, the resonance frequency is given by the equation       ω    0    =                    g        ⁢                  xe2x80x83                ⁢        β            ℏ        ⁢          (                        B          0                +                  G          ·          r                    )      
where, G indicates gradient vector, while r indicates the position vector in the sample.
The resolution of an EPR image is dependent on a number of parameters, including the intensity of the magnetic field gradient, the intrinsic width of the basic EPR resonance of the species in question (the xe2x80x98linewidthxe2x80x99), the signal-to-noise ratio per volume element of the sample, molecular diffusion processes, etc. In general, the EPR linewidth is of the order of several Gauss (1 Gauss xe2x96xa1 10xe2x88x924 Tesla), or several Megahertz in frequency units. The resolution R expected on the basis of the first two parameters may be given as:   R  =            Δ      ⁢              xe2x80x83            ⁢              v                  1          /          2                      G  
where xcex94vxc2xd denotes the linewidth and G, the gradient amplitude.
It is customary to generate the gradient by additional sets of anti-Helmholtz or Anderson coils located in the main magnetic field. When currents are passed through such water- or forced air-cooled coils, sizeable gradients may be generated, with amplitude upto about 1 T mxe2x88x921 (100 G cmxe2x88x921). One set of coils is typically employed for each of the three orthogonal directions in space. Because the inverse of the EPR linewidth is typically short compared to gradient switching times, it is customary to acquire the signal in the presence of the gradient, then reorient the sample with respect to the gradient (e.g. by rotating the gradient, adjusting the current amplitudes in two sets of gradient coils) to obtain a two-dimensional image. Reconstruction of the resulting series of xe2x80x98projectionsxe2x80x99 then yields the desired image.
As reported by Maresch et al (Physica B, 138, 261, 1986), it is possible, under special circumstances, to pulse the gradient and acquire ESR images as in Nuclear Magnetic Resonance (NMR) Fourier imaging. This entails the construction of a special, usually small, gradient coil assembly with low inductance, permitting short gradient pulse rise and fall times, the experiment being then performed with an ESR sample that gives rise to a single sharp resonance (the linewidth being of the order of 20-50 kHz, instead of being in the typical range of 1-10 MHz) in the absence of gradients.
The main objective of the present invention is to provide a device for Electron Paramagnetic Resonance Imaging, which obviates the special requirements, limitations and drawbacks stated above.
Another objective of the present invention is to provide a device for EPR imaging using naturally occurring field gradient.
Yet another objective of the present invention is to provide a device to use a field gradient of amplitude of minimum 5 T/m.
Still another objective of the present invention is to provide a device devoid of gradient coils, gradient amplifiers and the associated cooling system for generating EPR images.
One more objective of the present invention is to provide a device to image EPR samples like nitroxide radicals, having larger linewidth in the range of 1-10 MHz.
Yet another objective of the present invention is to provide a device for imaging on existing cw/pulsed EPR spectrometer/imaging systems, by the addition of a microwave bridge translation module, but without using gradient coils and amplifiers. Still another objective of the present invention is to provide a device to acquire ESR images in cw mode, as well as in pulsed mode of operation.
The novelty and non-obviousness of the present invention lies in the incorporation of microwave bridge translation module (33), whereby the resonator containing the sample can be placed, unlike in the conventional device where the sample placed in the resonator has necessarily to be put in the center of the magnetic field necessitating additional hardware and control system to generate the gradient, instead at a convenient optimal location in the field ensuring the optimal use of field intensity and the naturally occurring gradient without using the conventional hardware and control system for the imaging application.